The present invention relates to a control system for an infinitely variable belt-drive transmission for a vehicle, and more particularly to a system for changing the speed ratio or transmission ratio control characteristic. European Patent No. EP-A-0005565 discloses a control system for an infinitely variable transmission.
The infinitely variable belt-drive transmission comprises an endless belt running over a primary pulley and a secondary pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulley depending on driving conditions. The system is provided with an oil pressure regulator valve and a transmission ratio control valve. Each valve comprises a spool to control the oil by shifting the spool.
The primary pulley has a Pitot pressure generating device for producing Pitot pressure dependent on engine speed. The Pitot pressure is applied to one axial end of the spool of each valve to bias the spool. On the other hand, the actual transmission ratio is detected by the axial position of the movable conical disc of the primary pulley which represents the running diameter of the belt on the primary pulley. The position of the movable conical disc is transmitted to the other axial end of the spool of the pressure regulator valve by a rod and a link.
The spool of the transmission ratio control valve is shifted in dependency on the opening degree of a throttle valve of an engine and on engine speed to control the amount of oil supplied to the servo device of the primary pulley so as to control the transmission ratio. The spool of the oil pressure regulator valve is shifted in dependency on the engine speed, the pressure in the servo device of the secondary pulley and on the transmission ratio to regulate the pressure of the line oil. Thus, the transmission ratio can be smoothly and infinitely varied.
In the system, the opening degree of the throttle valve is transmitted to a cam, so that the lift of the cam may represent the opening degree of the throttle valve. Accordingly, the lift of the cam and the opening degree of the throttle valve is in constant relation. On the other hand, the relationship between the Pitot pressure and the engine speed is also fixed. Therefore, the transmission ratio control characteristic of the system is absolutely determined by the engine speed and the opening degree of the throttle valve regardless of driving conditions of the vehicle, such as flat road driving, uphill driving. Accordingly, the vehicle can not be driven at a suitable transmission ratio depending on driving conditions.
In addition, the Pitot pressure varies according to a second degree curve of the engine speed, in which the Pitot pressure does not change much in a low engine speed range. Accordingly, the control of the transmission ratio is not reliably performed out in a low engine speed range.